1. Technical Field
The invention is related to signal processing of received signals of the type having a set of allowable states or amplitudes, such as pulse amplitude modulated signals, such as signal processing employing equalization. In a particular application, the invention concerns the adaptive control of an equalizer employed in the processing of multi-state signals.
2. Background Art
Multi-state signals are employed in high speed (e.g., gigabit-per-second) network communications, such as local area networks of computers. While the present invention may find application in processing various types of multi-state signals, such as pulse amplitude modulated signals, phase modulated signals and so forth, the detailed description presented below concerns application of the invention to processing of pulse amplitude modulated signals.
Many high speed computer networks transmit ultra-high frequency signals (gigabit-per-second data) over a coaxial conductor cable. The cable introduces signal distortion, arising from certain characteristics of the cable such as its reactance. Signal distortion also arises in channels that do not employ an electrically conductive cable. Signal processing is employed to correct for such distortion. For example, the signal processing distortion correction may be performed by an equalizer of the type which introduces a certain reactance that compensates for the reactance of the cable. A conventional equalizer suitable for digital signal processing introduces a transfer function whose representation in the complex plane has appropriate poles and zeroes corresponding to the desired reactance, as is well known to the skilled worker. Various reactances may be stored in the equalizer, and one of them is selected at any one time. The problem is that the cable reactance is not known a priori, and therefore the equalizer must have a large number of settings (e.g., reactances) one of which is chosen only after actual testing in the field of the cable. Since the cable characteristics may not be constant and/or the cable may be changed by the user, the choice of equalizer setting must be made periodically during actual use of the network. This is accomplished by adaptive techniques in which the signal distortion is periodically or constantly monitored and the equalizer setting is periodically or constantly adjusted in a manner calculated to minimize the distortion.
Numerous conventional techniques have been employed to carry out such adaptive equalization. Such techniques include recursive algorithms such as a Recursive Least Squares adaptive algorithm and a Least Mean Square adaptive algorithm. A significant problem with such techniques is that these adaptive algorithms are mathematically intensive, involving large numbers of multiply and accumulate steps. Implementing a very large number of multiply operations in a circuit is very expensive and complex, making it difficult to provide such a product on a cost-competitive basis. Therefore, there is a need to provide adaptive equalization without requiring such a mathematically intensive algorithm or without requiring multiply and accumulate operations.